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Date: Thu, 14 Jul 2005 22:25:09 +0200
From: Matthias Buelow <mkb@incubus.de>
To: David Sze <dsze@distrust.net>
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Cc: freebsd-stable@freebsd.org, freebsd-questions@freebsd.org
Subject: Re: dangerous situation with shutdown process
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David Sze wrote:

>> Until a journalled fs that uses write request barriers is available
>> for FreeBSD, you better had a reliable UPS.
>
>How do OS-level request barriers help if the disk reorders pending
>writes in its cache?

By separating journal updates from the corresponding metadata (and/or
data) actions, and by guaranteeing (by flushing the cache, or a
singular disabling/enabling of the wb cache at the barrier) that
the journal is updated on disk before the actions take place. This
imposes an ordering on the journal vs. action requests, which is
what a journalled fs needs for filesystem integrity. It doesn't
really matter if the disk reorders writes within those two blocks,
the only thing that really matters is that the journal update is
completed before metadata (or data) updates take place. With
softupdates, as far as I understand, that doesn't work, because
there is no journal.  All requests must be in the order that
softupdates decrees. You'd have to issue a barrier request after
every write request, which would be equivalent to disabling the wb
cache.

mkb.